Those are not hallmarks and yes it’s definitely silver plate from John Round of Sheffield, England. Although established in 1847, their mark didn’t include ‘& S’ (for and Son) on plate until 1863.
Your set of marks also includes the ‘four trident’ trademark formerly used by Joseph Ridge, which puts it a little later. In 1886 Round’s business amalgamated with Ridge, Allcard & Co (also of Sheffield) with Joseph Ridge as general manager. Pieces from Round & Son that also bear Ridge’s tridents trademark are believed to have been made after that merger and so date from 1886 onwards, until perhaps as late as 1915.
Although it’s a nice piece, there really isn’t much market for these. You’ll see higher speculative pricing on fleabay, but you should be able to pick up a 4-piece set with tea and coffee pots (or one pot and a hot water jug) plus milk/cream jug and sugar bowl for about $75.